1. Field of the Invention
The present invention relates to a magnetic detection apparatus for detecting the strength of a magnetic field to find the position of a magnetic moving object.
2. Description of the Related Art
A typical example of such a magnetic detection apparatus has been known which is constructed as follows, for instance; magnetoelectric conversion elements in the form of magnetoresistance elements having electrodes formed at their respective ends are coupled with one another to provide a bridge circuit with a power supply of a constant voltage and a constant current connected with two opposed electrodes of the bridge circuit, so that a change in the resistance value of each magnetoresistance element is converted into a corresponding voltage change, which is then detected as a change in the magnetic field acting on each magnetoresistance element to thereby find the position of a magnetic moving object.
FIG. 16 is an electric circuit diagram of such a magnetic detection apparatus.
In this magnetic detection apparatus, a constant voltage is applied to a bridge circuit formed of a magnetoresistance element 1 and a fixed resistor 2, so that a change in the resistance of the magnetoresistance element 1 caused by a change in a magnetic field applied thereto is converted into a corresponding voltage change which is then amplified by an amplification circuit 3 and input to a comparison circuit 4. The signal input to the comparison circuit 4 is compared with a prescribed voltage so that it is converted into a final output signal of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d by means of an output circuit 5. This final signal is output from an output terminal 6.
FIGS. 17A and 17B illustrate the arrangement of a known magnetic detection apparatus. FIG. 17A is a perspective view thereof, and FIG. 17B is a partial plan view of FIG. 17A.
This magnetic detection apparatus is arranged in opposition to one of detected portions in the form of teeth 12a and grooves 12b of a magnetic moving object in the form of a rotating member 11.
The known magnetic detection apparatus includes a magnetoelectric conversion element in the form of a magnetoresistance element 15 having a magnetic resistance value which varies with a change in a magnetic field applied thereto, a magnet 14 with the direction of magnetization thereof oriented to the rotating member 11, the amplification circuit 3 for amplifying a signal in the form of a voltage change converted from a change in the resistance value of the magnetoresistance element 15, and a processing circuit 16 incorporating therein the comparison circuit 4 and the output circuit 5.
With the magnetic detection apparatus as constructed above, the rotating member 11 is caused to rotate in synchronization with the rotation of a rotation shaft 10, so that a magnetic field applied to the magnetoresistance element 15 from the magnet 14 is accordingly varied. As a result, the resistance value of the magnetoresistance element 15 changes between the time when a tooth 12a of the rotating member 11 comes to face the magnetoresistance element 15 and the time when a groove 12b of the rotating member 11 comes to face the magnetoresistance element 15, as illustrated in FIG. 18. Thus, the output of the amplification circuit 3 also changes accordingly. Then, the output of the amplification circuit 3 is waveform shaped by means of the processing circuit 16, so that the output terminal 6 of the processing circuit 16 generates a final output signal of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d corresponding to a tooth 12a or a groove 12b of the rotating member 11.
FIGS. 19A through 19C illustrate a change in a bias magnetic field applied to the magnetoresistance element 15 by the magnet 14 when the magnetoresistance element 15 is shifted or displaced from a predetermined position with respect to an opposed tooth 12a or an opposed groove 12b of the rotating member 11 in a radial direction thereof (i.e., in a rotating member facing direction, as indicated at arrow B, in which the magnetoresistance element 15 faces the rotating member 11), and when the magnetoresistance element 15 is shifted or displaced from the predetermined position with respect to an opposed tooth 12a and an opposed groove 12b of the rotating member 11 in a circumferential direction thereof (i.e., in a direction as indicated at arrow A). From FIG. 19C, it can be seen that the bias magnetic field changes more greatly when the magnetoresistance element 15 is displaced in the direction of arrow B, i.e., in a radial direction of the rotating member 11, than when the magnetoresistance element 15 is displaced in the direction of arrow A, i.e., in a circumferential direction of the rotating member 11.
FIG. 20 illustrates the respective operation waveforms of the resistance value of the magnetoresistance element 15, the output of the amplification circuit 3 and the final output of the output terminal 6 when the magnetoresistance element 15 is displaced in the direction of arrow B with respect to the detected portions in the form of the teeth 12a and the grooves 12b of the rotating member 11. In this figure, solid lines represent the operation waveforms when the magnetoresistance element 15 is in a normal position; dotted lines represent the operation waveforms when the magnetoresistance element 15 is out of the normal position; and alternate long and two short dashes lines represent comparison voltages.
As can be seen from this figure, the timing of position detection of a tooth 12a or a groove 12b of the rotating member 11 shifts by a period of time T1 when the magnetoresistance element 15 is displaced from its normal position. As a result, there arises a problem that the position of a tooth 12a or a groove 12b is not able to be detected accurately by a positional shift or displacement of the magnetoresistance element 15.
The present invention is intended to obviate the problem as referred to above, and has for its object to provide a magnetic detection apparatus which is capable of ensuring good-performance in detecting the position of a detected portion even if there is a shift or displacement in the position of a magnetoelectric conversion element.
Bearing the above object in mind, according to the present invention, there is provided a magnetic detection apparatus comprising: at least one magnetoelectric conversion element disposed to face a detected portion of a magnetic moving object; a magnet with a direction of magnetization thereof oriented in a direction perpendicular to a radial direction of the magnetic moving object in which the magnet faces the magnetic moving object; and a magnetic guide having at least two pole projections formed in a spaced apart relation with respect to each other in a radial direction of the magnetic moving object in which the magnetic guide faces the detected portion of the magnetic moving object. The at least one magnetoelectric conversion element is disposed between the pole projections when viewed from a direction perpendicular to the radial direction of the magnetic moving object in which the magnetic guide faces the detected portion of the magnetic moving object.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the invention taken in conjunction with the accompanying drawings.